Plasmas have an index of refraction less than 1. So say you sent one beam of light through a plasma and another one along a path just as long but in free space. Is the plasma beam's phase lead or lag the free space beam's phase?
In classical plasmas, light waves with a frequency higher than the plasma frequency have a phase velocity higher that the speed of light in vacuum $c$ (see this). It means that the phase leads compared to light in vacuum. The group velocity, however, is lower than $c$.